Modulating system



F. MOHRv April 12 1927.

MODULATING VSYS TEM Filed Jan. l5. 1925 Patented pr. 12, 1927.

EEaNxLm nona, or NEw- YoEx, N. Y.,

'assIGNoE `BY' M EsNE ASSIGNMENTS, 'ro

WTESTERNl ELECTRIC COMPANY, INCORPORATED, CORPORATION E NEW YORK.

MoDULaTmG SYSTEM.

Application led January v125, 1925. Serial No. 2,486.

This v'invention ,relates to translating circuits, and more particularlyto the circuits Y o'f space discharge inodulatlng devices.

A11 object of the invention isA to increase the out ut of useful energyof amodulator, preferahlyof the space discharge type.

A feature of the invention is the combination with the modulating deviceof load or output circuit having an impedance which varies with the wavefrequency according toy cial mode. ther objects of the invention and theprinciple of its "o erationwill be clearlyv understood from t efollowing detailed description when read in connection with the Aaccompanying drawings, ofv which Fig. 1 shows in schematic form amodulating system embodying the invention; l

K Fig. 2 shows attenuation characteristics relating to the system ofFig., 1; and

Fig. 3 shows impedance characteristics corresponding to the attenuationvcharacteristics of Fig. 2.

' The system shown in schematic form in Fig. 1 includes the necessaryapparatus for f preparing' and transmitting a single side bandof highfrequency waves for the purpose of radio tele hone communication by theso-called homodyne method. In accordance withthis method,a highfrequency carrier wave is modulated by speechV waves and fromthemodulation products one sideband corresponding to the speech waves isselected for transmission, the other side band and the carrier wavebothAbeing suppressed by suitable selective circuits; At the receivingpoint, a local generator supplies an auxiliary Vwave of the properfrequency, usually that of the suppressed carrier, which is impressedalong with the received Waves upon a demodulator, thereby enabling thetransmitted signals to be properly detected.

The advantages accruing from the method are principally an economy ofpower due to the restriction of the number of waves trans` Y mitted, amore eieient use of the frequency spectrum due to the elimination of oneside band, and, in the case ofradio signaling, a certain degree ofsecrecy vdue tothe absence of the carrier wave. Y f Y As illustrated,the system/ comprises a balanced space discharge modulator 1, of

the type disclosed inU. S. patent to J. R.

Carson, No. 1,343,307, issued June 15, 1920, upon the input'terminals ofwhich are impressed s'peech frequency waves from microphone 2 and acarrier'wave from a source 3.

` The products of `modulation are impressed .through `a balanced out uttransformer 4 upon a high-pass wave lter' 5, the. cut-off frequency ofwhich is so adjusted that only those waves correspondin to the sum ofthe two input fre uencies, t at is, theI higher frequency sideand waves,are freely transmitted. This filter maybe of the well known typ'edisclosed vin U. S. patent to G. VA.. Campbell, No. 1,227 ,113, issuedMay 22, 1917,

Y. but 1s preferably a composite filter, having "a very great'attenuation at frequencies close tothe cutfotl frequency. Filters ofthis general type are described by O. J. Zobel,

Theory and Design of Composite Electric Wave Filters, Bell SystemTechnical Journal, Vol. II No. 1,-January, 1923. The-balancedl modulatorin accordance withthe VCarson patent mentioned above delivers to the`filter only the side-band modulation products` vand those amplifiedwaves corresponding to the speechinput, the amplified 'carrier waveVbeing balanced out by thewind- -ings of transformer 4. Filter 5 isthereforerequired to suppress only the speech waves,

products, and` such smallfraction of the Vcarrierwave as-may resultfromimperfect y tions of the output transformer balance.`

The selected side-band isimpressed upon asecond space dischargemodulator 7 through transformer Gand is modulatedtherein with a secondcarrier Wave supplied froml source 8 through transformer 9. A band-passfilter 10 selects from the outputV of modulator 7 one bandof sidefrequencies and transmits seA 'the lower side-band of ythev modulationthem to' an amplifier 11, which may be of anyV well known type, lfromthe output of which wavesv of amplified energy are de-.

livered to a radiating' antenna 12. Filter 10, as illustrated, consistsof a single mid'-shunt. terminatedsection` but additional sections maybe used if desired.

The first step of modulation in modulain the frequency'spectrum close tothe carrier wave. The selection of a single sidetor 1 producesside-bands of waves which'lie band, while at the same time the other issubstantially sup ressed, requires a. sharply discriminating se ectivityin the filter 5. In practice, ithas been found that such selectioncannot be efiiciently accomplished if the frequency of the carrier waveis much above 40 kilocycles er second;

The use o a carrier wave of about 30 kilocycles per second frequencyrovides, as the result of the first step of moiiiilation and selection,a band of waves of frequencies ranging from 30 kilocycles per second upto about 35 kilocyclcs per second, and these waves when modulated in thesecond modu' later 7 with the second carrier wave produce sidebandswhich are separated from the carrier wave by a frequency difference ofat least 30 kilocycles. The second wave may be of comparatively highfrequency, for example 10() kilocycles per second, without imposing uponthe filter 10 any severe requirements as to selectivity.

The problem of preparing a single sideband, or a single band of puremodulated waves, involves not only the problem of selection as outlinedin the foregoing, but also that of securing the maximum power outputfrom the modulating devices.

A mathematical theory of modulators, having particular reference tosimple space discharge modulators of the three-electrode type7 iscontained in U. S. patent to J. R. Carson, No. 1,448,702, issued March13, 1923.

The following formulae are given in the above noted patent for the sidefrequency curi-cnt eomponents of the products of the modulation in aspace discharge amplifier and E -E 2 .2 IUQHn-MRO) Znfzruml.) (2) thevarious factors ofwhich are defined as follows:

E1 and lil2 are respectively the amplitudes of the carrier wave and thesignal wave elec tromotive forces impressed upon the modulator inputterminals, both waves being assumed to be sinusoidal;

`Zf, and Zr, are the impedances of the modulator output circuit,including the internal resistance of the modulator, corresponding to thefrequencies f, and f2 of the carrier wave and the ysignal waverespectively; Zmm) and Z( are the corre spending impedances at thefrequencies of the modulation product components, that is, at the upperand lower side frequencies;

n is the amplificationrfactor and Ro is the internal resistance ofthemodulator. The factor M is a constant the value of which depends uponthe rate of variation of the resistance Ro with the anode potential ofthe modulator in accordance with the following equation The power outputof the modulator in a given side-band, for example the lower frequencyband, is equal to the square of the current component of that frequencymultiplied by the resistance of the connected output circuit at thatfrequency. From equation (1) it is evident that the side-band powervoutput may be expressed by Ruz-h).

in which P is the power output, and Rukh, is the resistance of theconnected output circuit at the frequency (ff-f1). The factor' Kinvolves the coefficients, p, R0, and M, and the voltage E, and is aconstant so lon as the voltage is constant.

rlghe power output being determined in part by the load resistance, theimpedance conditions necessary to produce the maxi mum power may bedetermined by computing the partial derivative of the foregoingexpression for the sideband power with respect to the load resistance,and equating the result to zero in the usual manner. In this Way, thefollowing equation is found which states a condition for maximum,

power:

ZUn-f) A RUa-Ii) Z( are each equal to that is, to the internalresistance of the modulator plus the constant resistance ofthe connectedoutput circuit.

Substitution of this value inequation (5) gives the solution 1 R(f,m=5RyV(7) the partial derivatives beingr all equal to unity.

From equation (4) by means of equations l quency that accords with .therequirementsequations (6) and (7 the maximumL powerisy found Uaae 2. Theload impedance is zero `to wavesof frequency f1 and is uniformly relsistive to waves of the other frequencles.

The impedancesare v Zn' Rd Za Zon-n) Re l'lRua-m (9) `The partialderivative of Z', in equation (4) is zero and the other-partialderivatives.

are each unity. The solution of equation (5) obtained when the values ofequatlons (9) are isubstituted therein is which corresponds to a maximumpower Oase 3. The load impedance is zero tpwaves of both frequencies 'f1and f2 and 1s resistive at the -side band frequency.

The impedances are zl--a-.R- t Q 0 Zas-h) R0 fi' BH2-h) l Thesubstitution' of the values defined by for which condition 'i the givenby equation (4.)v is' The maximum powers obtainable yi n 4 the threecases investigated are the p1 oporindicating that a very great increaseinside band power may be obtained if th connected output impedanceis sodesignedv as to have' a characteristic variation with frestated inconnection with the thirdcase. The modulating systems-of the presentinvention lembody 'features whereby this increased power out ut isobtained.

In the system o Fig. 1 the conditions giving rise to the optimum poweroutput front second modulator 7 are obtained by the cooperation' of twofeatures. The first is the use of a mid-shunt termination of theband-pass filter 10 in theoutput-circuit of the modulator for the'selection of the proper side band and the suppression of the otherwaves.

(12) inequation (5) leads tothe power output nsare indicated bythefrequencierf. and fb.

Between these limits the im' edancc 1s resistive; infinitely great atthe imits but falling rapidly to al fairly uniform value throughout thegreater part of the range..

Outside the limits the impedance is reactive. and falls rapidly .toa lowvalue as the frequency decreases; above the upper limit it is capacitiveand falls rapidly to zero as the frequency increases. By well :knownmethods the filter may'be so proportioned that the resistance betweenthe band limits is equal to the modulator internal resistance for asubstantial range of frequencies. A possible relationship between thetwo resistances is illustrated in Fig. 3 by the curved line B,representing the filter resistance and the straight line DB1representing the modulator resistance. t

Figs. 2 and 3 show also the characteristics It is inductivebelow thelower limit o fv thefilter 5, the' attenuation characteristic beingrepresented in Fig. 2 by the discontinuous curve G,=the discontinuity of'which corresponds to t e resonance of the antiresonant Acircuit 13 inthe series terminal branch of the filter, and the impedancecharacteristic being shown 'in Fig. 3 by the discontinuous -curve H1 H2H3. The various wave frequencies involved in the production vof theinalside band are also indicated.

In modulator 1 a wave of frequncy fs -which may be considered as onecomponent of a complex speech vwave modulates a carrier wave from source3 having a frequency less than the cut-off frequency of filter 5. fAmongst the products `of modulation is a wave of frequency f1(=fc|fs),which is transmitted through the lter to the input of modulator Z inwhich it is applied to modulate the second carrier )2, supplied fromsource 8.

As may be seen from Figs. 2 and 3 itis necessary that the secondmodulation product of frequency faQ-f1 should fall between lic , f.which is preferably equal to or slightly the limits f. and b `of theband filter` 10,in

consequence of w ich .f2 must lie well outside these limits. lherelative posit-ions of these waves in the' frequency spectrum areclearly indicated in the diagrams.

In the load circuit the waves of frequencies f1 and f2 encounterpractically zero im'- pedanc'e, whereas the side-band waveof frequencyffl-f1 encounters a resistance practically equal to that of themodulator tube. 'The conditions are therefore correct to obta'in thegreatest possible amount of power from the modulator. It should be notedalso that the second carrier wave bein well outside the transmissionrange of t e filter is strongly. attenuated thereby.

The action in the output circuit of modulator 1 is of a similarcharacter but is slightly different in detail. Filter 5 is terminatedmid-series instead of-midsl1unt in consequence of which the impedance iszero at the cut-off frequency, instead of being infinite as in the casefor mid-shunt termination. By setting the frequency of the first carrierwave equal to or slightly lower than the cut-off frequency the desiredcondition that the load impedance be zero to t-he carrier wave isattained. If an ordinary Campbell type high-pass filter were used theimpedance would, on account of .the series capacity, increase towardsinfinity at low frequency, and the condition of low impedance to thesecond input wave f, could be attained only by making the impedance oftransformer 4 sufficiently low at speech frequencies. However, the useof the special termination shown in the ligure produces an impedancecharacteristic which falls to zero at very l'ow frequencies and therebysecures the desired condition of zero impedance in the load circuit toboth of the input waves.

The use of a balanced type modulator for the first stage of modulationlsecures by the neutralizing action of windings 'of the out`-' puttransformer 4, an output impedance which is effectively z ero at thefrequency `of the carrier wave, and at the same time substantiallysuppresses the carrier wave. `In a modulator of this sort the ellect ofthe load impedance characteristic is therefore principally to supplementthe action of the output transformer at the carrier wave frequenc and toprovide the lowest possible impe ance for the Waves of speech frequency.l

If instead of a balanced modulator as shown, a si'm le modulator, suchas is used for the secon step, were employed the filter would' stillprovide the desired impedance characteristic by which the maximum poweroutputis attained. The carrier wave, however, would not be completelysuppressed but would be partiallytransmitted with the side of a smallerfrequency range for the com munication channel.

The system shown in Fig. 1 may be made to operate in accordance withthis method by opening switch 14 in the output circuit of one modulatorthereby rendering that modulator inoperative.

In each of the modulation systems described a unique maximum poweroutput is obtained by so arranging the connected load circuit that itprovides a short circuit or zero impedance path'to the amplified wavescorresponding to each of the input waves, and that it provides aresistive path for the modulation products, equal in resistance to themodulator space path. By the use of a broad band filter structure theequality of the modulator and the load resistance is maintained over asubstantial range of flequencies thereby enabling a band of signal wavesto be efficiently transmitted.

The operation of a space discharge modulator in producing the maximumslde band power may be described very briefly as follows z--when theamplified currents corresponding to the impressed waves are permitted toattain their maximum intensity in the space discharge path, the maximumside band electromotive force is generated therein, and the maximum sideband power output is delivered to a load resistance when that resistancehas the same magnitude as the internal resistance of the modulator, orside band enerator.

The m of operation is not peculiar'to space discharge modulation but isbroadl applicable to modulating systems in genera In the space dischargemodulator the fea.- ture that causes the generation of the modulationproducts is the variability of the Space path resistance according tothe intensity of the impressed forces. In modulators generally, it isthe variability of the impedance of some particular element inaccordance with the intensityl of the waves by which it is traversedthat results in the conditions of the familiar generator lawarefrequencies to the second modulator and/ fulfilled.

would appear in the output of the antenna 12 in conjunction with tln`final side band. l The radiated wave would then no longer consist ofpure modulated waves but would be accompanied by a carrier wavecomponent, enabling the signals to be detected by simple detectors. Theadvantages of single side band transmission are, however, retained,

namely an economy of power and the use What is claimed is:

1. A modulating system comprising in combination, a space dischargemodulating device having an anode, a cathode and a control electrode,means for impressing simultaneously upon said control electrode acarrier wave and a band of signal waves, whereby,there are generated inthe space path of said device side bands of waves en. eration of themodulation products, an in iso having frequencies differing from thecarrier wave requency by the fre uencies of .the signal waves, and abroad and wave filter connected to said anode and said cathode adaptedto transmit. selectively one of said side bandsto a loadimpedance, saidWave vfilter being so arranged as to presentdevice having an anode, acathode, and a control electrode, means for impressing simultaneouslyupon said, control electrode a carrier wave and a band lof signal waves,whereby there are @generated in the space path of said device side bandsof waves having frequencies differing from the carrier wave frequency bythe frequencies of the signal waves, and a broad band wave filterconnected to said anode and said cathode adapted to transmit selectivelyone of said side bands to a load impedance, said wave filter beingterminated at the junction with said device by a shunt impedance branchcomprising an induetance and capacity connected in parallel and resonantat substantiall the frequency of said selected side ban s.

3. In a wave transmission System, asource ofvspeech waves, a source ofintermediate frequency carrierwaves, means for modulating said carrierwaves, by said speech waves, means for selecting from the output r10isaid 'modulation means a single inter- 40 mediate frequency side bandVcorresponding to said speech waves, a source of high frequenc. carrierwaves, a space discharge modu ating device; means for im ressingthereon' to be ii'iodulated said high frequeiicy and said selectedlintermediate frequenc side band, whereby there are gen-v erate in saiddevice side band waves of the summation and of the diereuce frequenciesof said impressed waves, and an output circuit connectedl to saidmodulating device including a broad-band wave filter, said filter beingadapted to transmit with maximum power waves of one of saidcgeneratedside bands and to present to waves of the frequencies impressed upon thespace' discharge modulator an impedance that is substantially zero.

4. AIn a wave transmission system, a source `of speech waves, a sourceof intermediate frequency carrier waves, means for modulating saidcarrier waves by said speech waves means for selecting from the productso said modulation a single intermediate frequency' sidefbandcorresponding to said speech waves, a :source of high frequency carrierWaves, alspace discharge modulating device, means for impressing thereonto be modulated said high frequency and said selected intermediatefrequencyside-band whereby there are enerated in said ,device side bandvwaves o the summation and of the difference' frequencies of saidimpressed waves, and an outputcircuit connected to said modulatingdevice including a broadband wave filter, said filter bein ranged astoprovide substantially s ort-circuit pafhs for waves of the impressedfrequencies repeated by said spacedischarge modulator and to selectivelytransmit waves of said difference frequenc side hand, the impedancethereto being su stantially equal to that of said modulating device.

5. In combination with a space discharge wavemodulator an output circuitincluding a broad band wave filter, said filter being so arrangedA as toprovide short circuit paths for waves repeated by said modulator and totransmit lectivel wave modulation products of predetermined frequencies,offering 1925. y FRANIQIN MOHR.

